MP 1,1-5,1 Mac Pro 5,1 Boot Loop Fails to Power Off

[O'Draighnain]

Hello everyone, I have a Mac Pro 5,1 (mid 2012 4-core) that I picked up a while ago. I use it (used it) to interface with legacy ADDA converters and recording software. It failed a while ago (wouldn't start). Long story short, I took it to a repairer who couldn't get it started said junk it (I live in NZ in the provinces on the edge of nowhere). I thought I'd give replacing the back plane a go and that worked and it was up and running again. I then did not use the computer for several months (I was finishing building a console) and when I tried starting it up a boot loop issue appeared.

I thought it might have been triggered by dust.

I can sometimes log in and it runs for a little while, but other times it starts, fans work, and then it directly enters the loop. I literally have to pull the plug to stop the boot loop.

I have tried replacing all of the RAM, replacing the CPU and replacing the power supply (as of today). None of these things appear to be the source of the problem as the issue is still occurring.

It sometimes runs for a little while longer (chimes and gets through to log in) if left off for a couple of days, but this is not very predictable. I thought it might be leaky caps and was sure it was going to be the power supply.

Often, when I manage to log in, if I try to do something (like click on an icon so the computer does something) that triggers an immediate fail (shutdown and attempted reboot).

After I replaced the CPU I thought I was making progress because I was able to: reset the NVRAM, navigate in the computer a bit (e.g. look at the log - I should have grabbed them while I could have) and shut down properly after about 5 minutes of running.

I am thinking of the following next:

1. trying another (third) backplane;
2. removing the hard drive and trying to boot with another drive with a clean OS X (I have a friend who might be able to provide this); or
3. trying to get the computer on long enough to get a boot disk in (I have Snow Leopard on CD).

I do not have another Mac to test for faulty components.

I guess there is also the GPU tray to try replacing...

I know there are a lot of threads about boot loops and I have tried some fixes indicated but I'm starting to be less sure about where to go now.

Any advice on what to try next would be greatly appreciated.

Thank you.

 

[tsialex]

Did you verified the RTC battery voltage? Did you thoroughly check for oxidation everywhere?

Replace the main disk for a brand new SSD with Snow Leopard to Sierra, anything newer will require firmware updates. Run ASD and check for errors.

AHT/ASD:

You need to run AHT and ASD (if it's a cross flashed early-2009/MP4,1>5,1, you'll need to run mid-2010 versions) to find any errors.

You need an Apple OEM GPU (HD 2600XT, 8800GT, Quadro FX 5600, GT120, HD 4870/5770/5870) to run AHT/ASD. AHT/ASD can be used with PC flashed GPUs, but will fail mid test when the GPU sensors are checked. Non flashed PC GPUs won't work at all. You need to boot completely vanilla, no OC, to use AHT/ASD.

You can get AHT links and instructions on how to install it on this GitHub article.

AHT, you need 022-4831-A.dmg:

[URL unfurl="true"]https://github.com/upekkha/AppleHardwareTest[/URL]

The ASD version that you need is 3S149. The AHT instructions are valid for creating an ASD USB key and Apple provides a PDF with it.

ASD, you need 3S149:

[URL unfurl="true"]https://www.loremipsum.info/ia-tools-blog/[/URL]

For people that have an early-2009 cross-flashed with MP5,1 firmware, you will need to run the MacPro5,1 versions of AHT (022-4831-A) and ASD (3S149).

 

[O'Draighnain]

Thank you for the response.

Sorry, I forgot to mention that the battery was good (3.1 - 3.2V).

I also inspected the back plane and cleaned with isopropanol. There were a couple of areas where it looked like there might be a bit of oxidative film on the back side of the back plane following traces. The CPU PCB was a bit greasy around the edges of the heat sink, particularly the front area near the RAM. This could definitely be cleaned up further (I noted your comment on another thread about dust accumulation on the oily areas becoming conductive) but I am a bit wary about the delicacy of these boards. I'll take another look.

I also gave the chassis and heat sink a good clean with an air compressor.

The Mac is not flashed, the GPU should be OEM (it is an HD 5770 and is held in place by one of the fans). I'll look into running AHT and ASD.

 

[tsialex]

You can also remove the RTC battery and try to boot a known working install of 10.6.4 to 10.11.6. Be sure to get a known working drive/macOS install, used one of unknown state will only cause more trouble to diagnose.

When the RTC battery is removed, some things that depend on a working and correct clock won't work, like Wi-Fi or macOS installs.

If it the Mac Pro boots reliably without the RTC, we can narrow the problems to the Mac Pro BootROM, since powering up without the RTC battery temporarily bypass the NVRAM volume for the current session.